The present invention relates to a thermomagnetic recording method such as, for example, a thermomagnetic recording method using irradiation of a laser beam.
In the method to record information by thermomagnetic recording in a recording medium, from which information is reproduced by reading information bits (magnetic domains) formed thereon by virtue of magneto-optical interaction, the recording medium having a magnetic thin film formed of a vertically magnetizable film is subjected in advance to initialization, i.e., to a treatment to orient the magnetization in the medium into one direction perpendicular to the plane of the film, and thereafter, magnetic domains having vertical magnetization in the reverse direction to the initial magnetization are formed by heating the medium locally by irradiation of a laser beam or the like, and thereby, the information is recorded thereon as a binarized information bits.
In such a thermomagnetic recording method, when altering recorded information, a process must, prior to the alteration, be performed to erase the recorded information (the process corresponding to the above described initialization), so that a certain time is taken to perform the erasing process, and therefore, recording at a high transmission rate cannot be achieved. As countermeasures against that, there have been proposed various real-time recording methods in which overwriting is made possible and thereby the period of time for performing such an independent erasing process can be eliminated. Among such thermomagnetic recording methods executing the overwrite, hopeful ones, for example, are that applies modulated external magnetic field to the medium and that uses two heads, an erasing head as well as a recording head. In the method using modulated external magnetic field, the recording is performed as disclosed, for example, in Japanese Laid-open Patent Publication No. 60-48806 by applying a magnetic field with the polarity corresponding to the state of an input digital signal current to a recording medium, which is provided thereon with an amorphous ferrimagnetic thin film having an axis of easy magnetization perpendicular to the film plane, at its region irradiated by a temperature raising beam.
When it is attempted to achieve a high speed recording at a high information transmission rate by the above described external magnetic field modulation method, an electromagnet operating at the rate, for example, on the order of one MHz becomes necessary, and a problem arises that it is difficult to fabricate such an electromagnet, and even if it is fabricated, consumed power and heat generated thereby become huge, and therefore, it cannot be put to practical use. Meanwhile, the two-head method requires an extra head and the two heads must be located apart, and therefore, such a problem occurs that a heavy-load drive system is required and the system becomes uneconomical and unsuitable for mass production.
The present applicant earlier proposed thermomagnetic recording methods intended to solve these problems in Japanese Patent Application Nos. 61-194961 and 61-194962 (corresponding to U.S. patent application Ser. No. 87,440, filed Aug. 20, 1987 which is U.S./4,955,007, and European Patent Application laid open under the number of EP-A-257 530). The thermomagnetic methods proposed in these applications are such that use a thermomagnetic recording medium provided with a first and a second laminated structure of rare earth-transition metal magnetic thin films and switch and modulate, in accordance with information to be recorded, for example, of "0" and "1", a first heating condition to heat the medium to a first temperature T.sub.1 which is virtually above the Curie temperature T.sub.C1 of the first magnetic thin film and not reversing the sub-latice magnetization in the second magnetic thin film and a second heating condition to heat the same to a second temperature T.sub.2 which is above the temperature T.sub.C1 and sufficient to reverse the sub-lattice magnetization in the second magnetic thin film, with the medium applied with a required first external magnetic field, so that, in the cooling stage, the direction of sub-lattice magnetization in the first magnetic thin film is brought into agreement with the direction of the sub-lattice magnetization in the second magnetic thin film by virtue of first and second exchange coupling force, whereby recorded bits (magnetic domains), for example, of "0" and "1" are formed in the first magnetic thin film, and the sub-lattice magnetization in the second magnetic thin film is reversed by virtue of a second external magnetic field or by virtue of only the first external magnetic field at room temperature when the composition of the second magnetic thin film has been selected so as to have its compensation temperature between the second temperature T.sub.2 and room temperature, and thereby obtain the conditions to make overwriting possible.
Since, throughout the above processes, there is no need of performing a special process (taking a time) for erasing, a high transmission rate can be attained, and thereby, the problems involved in the above described two-head system or the external magnetic field modulation system can be solved.
The thermomagnetic recording method according to Japanese Patent Application No. 61-194961 will be described below. The recording of information, for example, of "0" and "1" in this recording method is performed, as shown in FIG. 1 which schematically indicates the above described magnetized states of the first end second magnetic thin films 1 and 2 with small arrows relative to temperature T, by providing, at room temperature T.sub.R, a state A with the directions of magnetization in both the magnetic thin films 1 and 2 oriented in one direction and a state B with the same oriented in the reverse directions to each other. And these records are obtained by application of the external magnetic field H.sub.ex to the medium and heating the same to the first and second temperatures T.sub.1 and T.sub.2 by laser beam irradiation. For example, a laser beam is first impinged on the position in the state A, with the intensity or time of irradiation of the laser beam modulated in accordance with the recording signal, so that the heating temperature T is brought to the first heating temperature T.sub.1 virtually above the Curie temperature T.sub.C1 of the first magnetic thin film 1 and causing no reversal of magnetization in the second magnetic thin film 2 under the influence of the required external magnetic field H.sub.ex. By such heating, the first magnetic thin film 1 exhibits a state C where it loses its magnetization but, when the laminated film of the magnetic thin films 1 and 2, after the heating has been finished, is cooled below the temperature T.sub.C1, magnetization is produced in the first magnetic thin film 1. In this case, since it has been previously adapted such that the exchange coupling force with the second magnetic thin film 2 is dominant, the direction of magnetization in the first magnetic thin film 1 is oriented into the same direction as that of the second magnetic thin film 2. Namely, the state A is produced whereby information, for example, of a "0" is recorded.
Otherwise, the heating temperature T is brought to the second heating temperature T.sub.2 beyond the above described temperature T.sub.1 and sufficient to reverse the magnetization in the second magnetic thin film 2 with the external magnetic field H.sub.ex applied. By performing such heating, a state D in which the first magnetic thin film 1 has lost its magnetization and the second magnetic thin film 2 has reversed its magnetization is brought about. But, when the heating is finished and the laminated film of the magnetic thin films 1 and 2 are cooled below the temperature T.sub.C1, the first magnetic thin film 1 is subjected to the exchange coupling force from the second magnetic thin film 2, whereby a state E, i.e., a magnetized state opposite to the original, initialized state, is produced but by virtue of a subsidiary external magnetic field H.sub.sub applied in the vicinity of room temperature T.sub.R, the direction of the second magnetic thin film 2 is reversed, and thereby, a magnetized state B with magnetic domain walls 3 formed between both the magnetic thin films 1 and 2, the state B being only different from the magnetized state A in that the magnetization in the first magnetic thin film 1 has been reversed, is brought about, and thus, recording of information, for example, of a "1" is achieved.
The recording of information of "0" and "1" is achieved by obtaining the state A and state B as described above. In this case, the light-intensity-modulated overwriting is applicable to both the state A and the state B. More particularly, by having any position of those in the state A and the state B heated to the temperature T.sub.1 or T.sub.2 past the state C, by virtue of selected temperatures T.sub.1 and T.sub.2 as described above, the overwrite of the state A or the state B corresponding to the information "0" or "1" can be achieved no matter whether the original state was the state A or the state B.
In the magnetic recording medium of the described structure, the surface between the magnetic thin films 1 and 2 forming the laminated film is under the influence of exchange energy, whereby the magnetic domain walls 3 are formed in the first state B. The domain wall energy .sigma..sub.W is expressed as ##EQU1## (A.sub.1 and A.sub.2, K.sub.1 and K.sub.2 are exchange constants and perpendicular magnetic anisotropic constants of the first and second magnetic thin films 1 and 2.)
As the conditions required for achieving the overwrite, the condition under which transition from the state B to the state A does not take place at room temperature (-20.degree. C. to 60.degree. C.) is given by EQU H.sub.C1 &gt;H.sub.W1 =.sigma..sub.W /2M.sub.S1 h.sub.1 ( 2)
Also, the condition under which transition from the state B to the state E does not take place is given by EQU H.sub.C2 &gt;H.sub.W2 =.sigma..sub.w /2M.sub.S2 h.sub.2 ( 3)
Further, in the state E, in order that the magnetization in the first magnetic thin film 1 is not reversed by the subsidiary external magnetic field H.sub.sub, the following condition must be satisfied: EQU H.sub.C1 .+-.H.sub.W1 &gt;H.sub.sub ( 4.sub.1)
where the sign .+-. on the left-hand side becomes sign "+" when the first magnetic thin film 1 is a rare earth metal rich film and the second magnetic thin film 2 is transition metal rich film, whereas it becomes sign "-" when both the first and the second magnetic thin films 1 and 2 are transmission metal rich.
Besides, in order that the transition from the state E to the state B takes place, the condition EQU H.sub.C2 +H.sub.W2 &lt;H.sub.sub ( 4.sub.2)
must be satisfied.
Further, where the heated temperature is in the vicinity of the Curie temperature T.sub.C1 of the first magnetic thin film 1, in order that the transition from the state C to the state A takes place, that is, the direction of magnetization in the first magnetic thin film 1 is brought into agreement with the direction of the magnetization in the second magnetic thin film 2, the condition EQU H.sub.W1 &gt;H.sub.C1 +H.sub.ex ( 5)
must be satisfied. Besides, in order that transition from the state B to the state E does not take place, the condition EQU H.sub.C2 -H.sub.W2 &lt;H.sub.ex ( 6)
must be satisfied.
In the above expressions, H.sub.W1 and H.sub.W2 are quantities defined by the expressions (2) and (3), and H.sub.C1 and H.sub.C2, M.sub.S1 and M.sub.S2, and h.sub.1 and h.sub.2 respectively are coercive forces, saturation magnetizations, and thicknesses of the first and second magnetic thin films.
As apparent from these, in order to satisfy the expressions (2) and (3), it is preferred that the domain wall energy .sigma..sub.W at room temperature is as small as possible, but, when assuming that K.apprxeq.4.times.10.sup.6 erg/cm.sup.3, A=2.times.10.sup.-6 erg/cm, we obtain EQU .sigma..sub.w .apprxeq.3.6 erg/cm.sup.2.
Meanwhile, actual measurements on the hysteresis loop of the two-layer film give .sigma..sub.W =3 to 6 erg/cm.sup.2. Now, assuming that .sigma..sub.W =5 erg/cm.sup.2 and using H.sub.c M.sub.s .apprxeq.0.45.times.10.sup.6 erg/cm.sup.2 and H.sub.ex =2 kOe, we obtain h.sub.2 =1100 .ANG., H.sub.C2 =4kOe, and H.sub.W2 .ltoreq.2kOe as approximate values to satisfy the condition of the expression (6) at room temperature T.sub.R, i.e., to satisfy the condition H.sub.C2 -H.sub.W2 &lt;2kOe. Thus, a problem is posed that the thickness h.sub.2 of the second magnetic thin film 2 becomes large and the subsidiary external magnetic field H.sub.sub becomes large from the expression (4.sub.2).